Alcohol by volume is a standard measure of how much alcohol is contained in a given volume of an alcoholic beverage. It is defined as the number of millilitres of pure ethanol present in 100 mL of solution at 20 °C.
In the British Imperial system of measures, the proof value of a beverage was equal to twice the alcohol by volume. While proof values are still often used in recipes and marketing materials, the ABV standard is now used worldwide.
Under laboratory conditions, the number of millilitres of pure ethanol is the mass of the ethanol divided by its density at 20° C, which is 0.78945 g/ml.
Note that in some countries, notably France, ABV is often referred to as "degrees Gay-Lussac". The actual measurement of the Gay-Lussac value is slightly different since it is performed at 15° C.